Apparatus for controlling Z-position of probe

ABSTRACT

Apparatus of easily controlling the Z-position of the probe used in a microprobe analyzer. The apparatus has: (A) a holder, (B) a reference body having a reference surface that is at the same height as a surface of a sample, the reference body being placed on or in the holder, (C) a probe-positioning device for bringing the probe into contact with the reference surface, (D) a controller for controlling motion of the probe-positioning device in the Z-direction, (E) position-measuring apparatus for measuring the Z-coordinate of the probe at which it is in contact with the reference surface, (F) a memory for storing a positional coordinate outputted by the position-measuring apparatus, and (G) probe contact detection apparatus for detecting that the probe is in contact with the reference surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to method and apparatus for controllingthe Z-position of a probe used in a microprobe analyzer.

2. Description of Related Art

In some microprobe analyzers, a mechanical probe is brought into contactwith the surface of a sample, and a tiny piece of the sample including acertain region of the substrate of the sample is separated and extractedusing an ion beam and the probe, thus preparing the sample piece formicroanalysis. Other microprobe analyzers are used to measure thecharacteristics of a sample while a voltage is applied to the surface ofthe sample with a probe. Such a microprobe analyzer is equipped with aZ-drive for bringing the probe into contact with the surface of thesample.

In a related art apparatus of this kind, information about the height ofthe probe from the surface of the sample is obtained based either on asecondary electron image in which a shadow produced immediately beforethe probe touches the sample surface is observed or on variations in thepositional relationship between a probe image formed when the ion beamis made to obliquely hit the sample and an image of the sample (see, forexample, JP2002-40107).

The above-described microprobe analyzer is equipped with a microscopemechanism for recognizing the portion of the sample to be observed and aportion of the sample with which the probe should be brought intocontact. Since the image created by the microscope mechanism is atwo-dimensional image, positions along the height cannot be recognized.Therefore, the operator causes the probe to descend toward the samplewhile observing the sample, thus bringing the probe into contact withthe surface of the sample.

However, these manipulations impose excessive stress to the operator.Furthermore, the probe may be struck against the sample surface,damaging the probe. Such cumbersome manipulations and damage to theprobe will eventually lead to a decrease in the throughput.

The probe is normally made of a hard metal, such as tungsten, and so ifsuch a probe comes into contact with a semiconductor sample of Si or thelike, a Schottky barrier is created in the probe. As a result, itbecomes difficult to electrically connect the probe with the sample.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide method and apparatusfor controlling the Z-position of a probe more easily than heretofore.

A first embodiment of the present invention provides a method ofcontrolling the Z-position of a probe, the method starting withproviding a reference body having a reference surface that is identicalin Z-position with a surface of a sample. The probe is brought close tothe reference surface. The Z-coordinate of the probe at which the probeis in contact with the reference surface is stored in memory. The probeis controlled to reach the stored Z-coordinate. Under this condition,the probe is brought into contact with the surface of the sample.

A second embodiment of the present invention is based on the firstembodiment and further characterized in that gold or platinum is used inor on the reference surface.

A third embodiment of the present invention is based on the firstembodiment and further characterized in that a strain gauge is used todetect that the probe is in contact with the reference surface.

A fourth embodiment of the present invention is based on the firstembodiment and further characterized in that the Z-position of the probeat which it is in contact with the reference surface is measured using alinear encoder.

A fifth embodiment of the present invention provides an apparatus forcontrolling the Z-position of a probe, the apparatus having (A) aholder, (B) a reference body having a reference surface that isidentical in Z-position with a surface of a sample, the reference bodybeing placed on or in the holder, (C) a probe-positioning device forbringing the probe into contact with the reference surface, (D) acontroller for controlling motion of the probe-positioning device in theZ-direction, (E) position-measuring apparatus for measuring theZ-coordinate of the probe at which the probe is in contact with thereference surface, (F) a memory for storing a positional coordinateoutputted by the position-measuring apparatus, and (G) probe contactdetection apparatus for detecting that the probe is in contact with thereference surface. In order to bring the probe into contact with thesurface of the sample, the probe-positioning device is moved until theZ-coordinate of the probe reaches the value stored in the memory.

(1) According to the first embodiment of the present invention, theZ-coordinate of the probe when it is in contact with the referencesurface that is identical in Z-position with the surface of the sampleis stored in memory. The probe is brought close to the sample to achievethe Z-coordinate. Consequently, Z-motion control provided by themicroprobe analyzer can be performed more easily.

(2) According to the second embodiment of the present invention, gold orplatinum is used in or on the reference surface. As a result, thereference surface is softened. If the probe comes into contact with thereference surface, damage to the probe can be prevented. Furthermore,when the probe is contacted with the reference surface, the gold orplatinum adheres to the tip of the probe. Consequently, when the probeis contacted with the sample, the probe can be electrically connectedwith the sample.

(3) According to the third embodiment of the present invention, contactof the probe with the reference surface can be judged from the outputfrom the strain gauge.

(4) According to the fourth embodiment of the present invention, theZ-coordinate of the probe at which it is in contact with the referencesurface can be measured using the output from the linear encoder.

(5) According to the fifth embodiment of the present invention, theZ-coordinate of the probe at which it is in contact with the referencesurface that is identical in Z-position with the surface of the sampleis stored in memory. The probe is brought close to the sample until thestored Z-coordinate is reached. Hence, Z-motion control provided by themicroprobe analyzer can be performed more easily.

Other objects and features of the invention will appear in the course ofthe description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a microprobe analyzer according to oneembodiment of the present invention;

FIGS. 2( a) and 2(b) are cross sections showing the manner in which theprobe shown in FIG. 1 comes into contact with a reference surface andwith the surface of a sample; and

FIG. 3 is a vertical cross-sectional view of a holder according to oneembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are hereinafter describedin detail with reference to the drawings.

FIG. 1 shows a microprobe analyzer according to one embodiment of thepresent invention. The analyzer has a probe 1, a probe-positioningdevice 2 to which the probe 1 is attached and which can move in threedimensions, i.e., in X-, Y-, and Z-directions, a holder 3, and areference body 5 attached to the holder 3. A sample 4 is also attachedto the holder 3. The sample 4 and reference body 5 are attached to theholder 3 in such a way that the upper surface of the sample 4 is flushwith the surface (reference surface) of the reference body 5. At leastthe reference surface of the reference body 5 is made of gold orplatinum.

The probe analyzer further includes a CPU (central processing unit) 6for controlling the operation of the whole apparatus. A memory 7 isconnected with the CPU and stores necessary data. A bus 8 interconnectsthe probe-positioning device 2 and the CPU 6.

The analyzer further includes an ammeter 9 for measuring an electricalcurrent flowing in via the probe 1, a voltage power supply 10, andanother ammeter 11 for measuring an electrical current flowing into thevoltage power supply 10.

A strain gauge 12 is mounted within the probe-positioning device 2 andused to detect that the probe 1 is in contact with the reference surfaceof the reference body 5. A linear encoder 13 that is mounted within theprobe-positioning device 2 in the same way as the strain gauge is usedto recognize the absolute position of the probe 1 in the Z-direction.

A control portion 14 for entering various commands is connected with theCPU 6. A coordinate input device, such as a keyboard or a computermouse, is used as the control portion 14. The operation of the apparatusconstructed as described so far is described below.

The probe 1 can be placed in position in the X- and Y-directions whilethe operator is making an observation with a microscope. However,positions in the Z-direction (along the height) cannot be graspedaccurately, because any coordinate system providing a basis cannot bedefined.

Accordingly, the tip of the probe 1 is once brought into contact withthe reference surface of the reference body 5 by a manual operation. Inparticular, when the operator supplies a control signal for Z-motionfrom the control portion 14 to the CPU 6, the CPU sends a driving signalto the probe-positioning device 2 via the bus 8. The probe-positioningdevice moves in steps in the Z-direction.

The probe 1 moves downward in the Z-direction in this way. When the tipof the probe 1 touches the reference surface, the output from the straingauge 12 varies. The output from the strain gauge is sent to the CPU 6via the bus 8.

When the CPU 6 recognizes from the output from the strain gauge 12 thatthe probe 1 is in contact with the reference surface, the CPU receivesthe present output from the linear encoder 13 indicative of theZ-coordinate of the probe 1 via the bus 8 and stores the Z-coordinate inthe memory 7. The stored value is used as a reference value. At the sametime, Z-motion of the probe-positioning device 2 is stopped.

Then, the operator sends a control signal to the probe-positioningdevice 2 via the CPU 6 from the control portion 14 to move theprobe-positioning device such that the probe 1 is spaced a givendistance from the reference surface.

Under this condition, the operator supplies a start signal to the CPU 6from the control portion 14. The CPU sends a control signal to theprobe-positioning device 2 via the bus 8. Then, the probe-positioningdevice begins to operate to bring the probe 1 into a desired position onthe sample 4.

Since the position of the probe 1 in the X- and Y-directions can berecognized from the XY coordinate system on the image on the microscope(not shown), the probe 1 can be moved into a desired position in the X-and Y-directions.

When the probe 1 placed above the target position on the surface of thesample 4 is moved downward in the Z-direction, the CPU 6 reads thereference value from the memory 7. The CPU compares the present value ofthe Z-coordinate sent in from the linear encoder 13 via the bus 8 withthe reference value read out to thereby calculate the distance betweenthe tip of the probe 1 and the surface of the sample 4.

The CPU 6 supplies the calculated value to the probe-positioning device2 via the bus 8. The probe-positioning device descends a distance basedon the calculated value in the Z-direction. As a result, the tip of theprobe 1 reaches the target position on the surface of the sample 4.

In this way, according to the present invention, the Z-coordinate atwhich the tip of the probe 1 touches the reference surface that isidentical in Z-position with the surface of the sample is stored inmemory. The probe 1 is brought close to the surface of the sample suchthat the stored Z-coordinate is reached. Consequently, Z-motion controlprovided by the microprobe analyzer can be performed more easily.

Furthermore, according to the present invention, at least the referencesurface of the reference body 5 is made of a noble metal (such as goldor platinum) that does not induce interaction with gaseous molecules.For example, the noble metal is deposited on the reference surface ofthe reference body 5 by vapor deposition. Therefore, when the probe 1 iscontacted with the reference surface, factors (such as dust andcontaminants) which increase the contact resistance can be removed.

In addition, the reference surface is soft, because the referencesurface is made of a noble metal in this way. Therefore, if the probestrikes the reference surface, damage to the probe can be prevented.

Moreover, when the probe is contacted with the reference surface, thenoble metal adheres to the tip of the probe and so when the probe iscontacted with the sample, the probe can be electrically connected withthe sample.

FIG. 2( a) shows the manner in which the probe 1 comes into contact withthe reference surface. FIG. 2( a) shows the state in which the tip ofthe probe 1 is in contact with the reference surface, while FIG. 2( b)shows the state in which the probe 1 is spaced from the referencesurface and is descending toward the sample 4.

When the tip of the probe 1 is contacted with the reference surface asshown in FIG. 2( a), the film of gold (Au) formed on the referencesurface peels off the reference surface and adheres to the surface ofthe probe 1 because the surface of the probe is rough. A film of gold 16is formed around the tip of the probe 1.

If a noble metal, such as gold or platinum, touches Si, no Schottkybarrier is formed. Therefore, electrical connection with the sample madeof Si or the like can be made.

The voltage power supply 10 and ammeters 11, 9 of FIG. 1 are used whenthe characteristics of the sample 4 are examined in practice. Thecharacteristics of the sample 4 can be measured by measuring either theelectrical current flowing into the probe 1 when it comes into contactwith the sample 4 or the electrical current flowing through the probewhen a voltage is applied to the sample 4 via the probe 1.

In the above embodiment, a strain gauge is used as the probe contactdetection apparatus. The present invention is not limited to this. Thefollowing means may also be used.

(1) Tunneling Current Detection Means

A bias voltage is applied between the sample and the probe. A tunnelingcurrent flowing when the probe is brought close to the sample ismeasured. Contact of the probe with the sample is detected.

(2) Capacitance Detection Means

A coil having an inductance of L, a capacitor having a capacitance of C,and a resistor having a resistance of R are connected in series betweenthe probe and the sample. An AC voltage is applied to this seriesconnection. The resulting resonant frequency is measured. Thus, contactof the probe with the sample is detected. When the probe is contactedwith the sample, an AC current flows through this circuit, maximizingthe resonant frequency.

In the above embodiment, a linear encoder is used as theposition-measuring apparatus for measuring the Z-coordinate of the probeat which it is in contact with the reference surface of the referencebody 5. The present invention is not limited to this. The followingmeans may also be used.

(1) Optical Detection Means

A laser light source is placed in a stationary reference positionrelative to a moving object. Laser light is shot at the moving object. Areflective mirror is mounted on the moving object. Reflected laser lightis detected. When the moving object is in motion, the optical pathdifference varies. The resulting interference distance is analyzed, andthe absolute position is identified.

(2) Resistance Detection Means

The position is identified by bringing a detection terminal into contactwith a resistor having a certain distance (length) and detecting theresistance between one terminal of the resistor and the detectionterminal. Since the resistance is in proportion to the length, theposition can be identified.

(3) Capacitance Detection Means

Where two objects are close to each other, a capacitance C producedbetween them is in proportion to S/D, where S is the area and D is thedistance between the objects. Therefore, the capacitance C varies withvarying the distance D. The distance D between the objects can be foundby measuring the capacitance C between the objects.

Furthermore, in the above embodiment, the sample 4 and the referencebody 5 are mounted to the holder 3 such that the surface of the sample 4and the reference surface of the reference body 5 are at the same heightin the Z-direction. The present invention is not limited to thisstructure. For example, a structure as shown in FIG. 3 may also beadopted.

Referring next to FIG. 3, a holder 21 of U-shaped cross section has arecessed portion. A sample stage 22 is disposed on the bottom surface ofthe recessed portion via springs 23A and 23B. Upper surface-limitingplates 24A and 24B are mounted to the upper surface of the holder 21. Areference body 25 in the form of a rectangular parallelepiped or cube ismounted to a jaw portion located outside the recessed portion of theholder 21. The reference body is so dimensioned that when it is mountedto the holder 21, the upper surface of the reference body as viewed inthe Z-direction is flush with the lower surfaces of the uppersurface-limiting plates 24A and 24B.

When the sample stage 22 has been drawn downward, the sample 4 is setbetween the stage 22 and the upper surface-limiting plates 24A and 24B.After the sample has been set, the sample stage 22 is ceased to bedrawn. As a result, the surface of the sample 4 is limited by the uppersurface-limiting plates 24A and 24B. Under this condition, the surfaceof the sample 4 agrees with the Z-position of the reference surface ofthe reference body 25.

The advantages of the present invention described so far are listedbelow.

(1) The reference point of the probe in the Z-direction is taken at thesurface of the sample. This facilitates setting the probe at thereference point. This can alleviate the burden on the operator.

(2) As the controllability is improved, the throughput is improved.

(3) It is easy to make a contact with a polysilicon part. Normally, itis difficult to make an electrical connection with such a polysiliconpart.

In this way, according to the present invention, method and apparatuscapable of controlling the Z-position of a probe more easily thanheretofore can be offered, the probe being used in a microprobeanalyzer.

Having thus described our invention with the detail and particularityrequired by the Patent Laws, what is desired protected by Letters Patentis set forth in the following claims.

1. An apparatus for controlling a Z-position of a probe, said apparatuscomprising: (A) a holder; (B) a reference body having a referencesurface that is identical in Z-position with a surface of a sample, thereference body being placed on or in the holder; (C) a probe-positioningdevice for bringing the probe into contact with the reference surface;(D) a controller for controlling motion of the probe-positioning devicein a Z-direction; (E) position-measuring means for measuring aZ-coordinate of the probe at which the probe is in contact with thereference surface; (F) a memory for storing a positional coordinateoutputted by the position-measuring means; and (G) probe contactdetection means for detecting that the probe is in contact with thereference surface, wherein in order to bring the probe into contact withthe surface of the sample, the probe-positioning device is moved untilthe Z-coordinate of the probe reaches the value stored in the memory.